PHYSIOLOGICAL DEMONSTRATION 



239 



simple apparatus, making the scale c c' of glass, the different 

 effects, both in kind, and roughly in degree, of continuous, 

 making, breaking, or intermittent currents can be pro- 

 jected. 



127. Blood-pressure. Most experiments of this class can 

 be demonstrated upon the screen, preparatory to exhibiting 

 their tracings in the same manner. Fig. 126 is a general 

 sketch of Ludwig's Kymographion, which makes use of the 

 mercury or u^ube manometer. The normal level of the 

 mercury is da. In the blood-vessel cc is tied a y-shaped 

 end of the short limb of the U-tube, the space between the 

 blood-vessel and the mercury being filled with a solution of 

 sodic bicarbonate to prevent coagulation. To the surface of 

 the mercury in the long limb Ludwig ap- 

 plied floats, bearing at the top an arm / 



carrying a tracing-point which makes the 

 trace on the blackened plate c as the pres- 

 sure of the blood depresses the mercury in 

 one limb to a variable point a' and of course 

 raises the column in the other tube to b. 

 We have only to remove the clockwork and 

 blackened plate, and if desirable the float 

 and style as well, and place behind the 

 manometer a black board or card with a slit 

 cut in it sufficiently large to display the 

 long tube. The tube and slit being then focussed, the rise 

 and fall of the mercury will distinctly appear upon the screen, 

 either inverted, or correctly if an erecting prism is 

 Fick and Bourdon's instrument, employing a sprii 

 of the mercury column, may be projected witl 



128. Electric Currents. The currents 



as in muscle or nerve can be demonstp^d^i the 

 more than one way. In many instances th< 

 itself into the simple projection of a galYanQ^et'er, 

 ing galvanometer may be employed ; p\pjeciti^ wSo if < 



j-vj vuo ,-> uj 



ft 



FlO. 126. Ludwig's 

 Kymographion 



